Should I just get a jump and then drive it around to charge the battery? Or should I buy a new battery? Or buy a battery charger and charge up my dead battery? I have heard that using the alternator to charge a battery is bad for the alternator. Is this true?

My understanding is that car batteries are designed to provide high current for brief time to start the car, but not to provide it for long periods. Your battery will probably have a decreased life at this point.

With a battery that has been that far discharged, if you just jump it and let the alternator charge it you will probably end up with a battery that dies again fairly quickly, as -D/a pointed out. You will do much better in the long-term if you use a proper battery charger.

I'm not sure I'd recommend it (especially when good battery chargers for car batteries are readily available) but I've had a lot of success using an inexpensive motorcycle battery charger that I had originally purchased for a battery that I was putting in a robot. It charges the battery much more slowly, which allows the charge to distribute more evenly through the metal plates.

With the ignition at the "key on" position...the radio was running...is this a high current state?

Ok half hour. Thank-you.

It's not so much the current draw, but the depth of discharge; 2.47 volts is far below the point at which damage starts (10.5 volts). If it were a lithium-ion battery and not lead acid (not that any standard cars use them), it would be completely ruined and dangerous to try to recharge; not sure if there is any risk with lead acid (the link above says that the battery can be saved if you immediately recharge it (the time you have not given), otherwise the lead sulfate crystallizes and can't be broken down).

Get a small 1 to 2 amp charger that has a maintainer circuit built in. These units are designed to be attached to a battery and left connected. If you are going to have a automobile, you should have one of these. IMO, all cars should come with such a device built into the electronics of the vehicle so you could just plug it in. But, it would add $10-15 to the cost of a $30,000 item, so they leave them out.

Charging a severely discharged battery with the vehicle's alternator is bad for the alternator and the battery. The charging rate is too high and the current draw on the alternator is higher than normal. Not that this will kill them immediately, but if you can wait and charge the battery with a battery charger, it would be best. One note, charging a dead or nearly dead battery will kill a brand new alternator.

Like I said, you really need a small charger (keep it in the trunk if this is your only vehicle) and use it to charge your battery if you can. If you have the car sit for more than a week or so, it would be best to charge it for a day or so before starting it. Particularly with today's electronics in vehicles, a fully charged battery is a good thing.

Whether or not to pro-actively replace the battery? I wouldn't. I would pay attention to the battery after being recharged and at the first sign of a problem, replace it, but if it is working fine, it will very likely come back with no noticeable damage. They don't like to be deeply discharged and really don't like being deeply discharged for days or weeks at a time, but I wouldn't replace it if it was performing just fine.

Should I just get a jump and then drive it around to charge the battery? Or should I buy a new battery? Or buy a battery charger and charge up my dead battery? I have heard that using the alternator to charge a battery is bad for the alternator. Is this true?

It depends.
How old is the battery? If it is near the endof its life it may not hold the charge. The alternator should be able to charge a flat battery with out damanaging itself.

To bring back to llife an old battery the slower the charge the better.

If was me, or when I had this problem I made sure the terminals were cleaned, jump started the car. If the battery holds the charge no problem, but if it does not prepair to replace it soon. Run around and charge the battery. But I would not stop anywhere that a failure to hold charge would be a problem. Use your fluke test the voltage at an idle, the rev the engine and retest. It should be a little higher. Shut the engine off, test the voltage. Wait 10 minutes retest. It should only drop 0.1 to 0.3 volts. If it is more then recheck 30 minutes later. If it is still dropping by much prepair to replace.

Regardless of the method you use to charge the battery, assuming that it is the type of battery that allows this, you should check the level of liquid in it and add water if needed, once it is charged.

I've had it happen twice. Both times the batteries drained because of a dead alternator. I replaced the alternator and the battery worked for about a week then stopped holding a charge. Since then, whenever I've had a deeply discharged battery like that, I've used an external charger and haven't had a problem.

Your basic lead-acid battery is a lead plate and a lead oxide plate immersed in an electrolyte consisting of water and sulfuric acid. As the battery discharges, the lead and lead oxide plate both transition to lead sulfide, and the sulfuric acid transitions to water. Charging reverses it.

I haven't done a lot of research on it, nor have I cut a battery plate in half to see what's going on, but I suspect that when you quickly charge the battery (like with the car's alternator) the lead sulfate to lead and lead oxide reactions occur mostly on the surface, leaving lead sulfate underneath. The battery won't hold as deep of a charge, and if the sulfate builds up closer to the surface then then the battery won't even hold a charge on the surface of the plates any more either. This is a bit of a WAG on my part though.

I'm with Michael63129 and engineer_comp_geek. If you're truly reading only 2-1/2 volts DC, the battery is toast. A six-cell lead-acid battery is considered fully discharged at 11.7 volts open-circuit. At 2.5 volts, the plates are surely sulfated so badly that no charger is likely to break through that.

You could slow-charge the battery with a battery charger. I'd be looking for a 2 amp charging rate. I've seen batteries that got that low take a charge and work for a good while if they were slow-charged.

Using an alternator to charge a battery that low is not wise. It's very tough on the alternator and likely will not get the battery properly and fully charged.

Alternatively, replace the battery. If its age is 3 years or more, this is probably the best choice.

To agree with excavating, your investment in a battery charger would be something which will return your purchase many times over. To agree with other posts, running your cranking battery down to that level has certainly severely shortened its life. If you need to rely on the car, get a new battery too.

I haven't done a lot of research on it, nor have I cut a battery plate in half to see what's going on, but I suspect that when you quickly charge the battery (like with the car's alternator) the lead sulfate to lead and lead oxide reactions occur mostly on the surface, leaving lead sulfate underneath. The battery won't hold as deep of a charge, and if the sulfate builds up closer to the surface then the battery won't even hold a charge on the surface of the plates any more either. This is a bit of a WAG on my part though.

I think you're on the right track. And today's high CCA batteries tend to be susceptible to sulfation-induced damage due to the use of very thin plates. (High CCA → low internal resistance → lots of surface area → lots of plates → thin plates.) In other words, for a given volume there's a tradeoff between high CCA and susceptibility to sulfation after a discharge, all else being equal.

I went into advance autoparts. The tech said that he could charge it at 10amps. It would take about a half hour. Also would run a test to see if it was OK....but after reading about the likely damage I think it's best to replace it as its my wife's car...used to cart around the kids.

but after reading about the likely damage I think it's best to replace it

Yep.

Last Sunday my 13 year old daughter was cutting the grass with our riding mower. After a few hours she came in the house and said, "The mower ran out of gas."

I didn't think anything of it at the time. The mower sat there until Friday, upon which I finally got around to filling up the gas can and filling the mower with gas (see my related pit thread). It was then that I discovered she had left the mower's ignition switch in the "ON" position after it ran out of gas. Battery was dead. Which meant the battery was dead for a five days.

Even though I have an external battery charger, I didn't even bother trying to charge it. A lead acid battery that remains in a discharged state for more than a day or so is likely toast. Went out and bought a new one.